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The International Union for Conservation of Nature added the Eastern lowland gorilla (Gorilla berengei) to its list of critically endangered species Sunday, raising the number of great ape species that are on very cusp of extinction to four.

The three other critically endangered species of great apes are the Western gorilla (Gorilla gorilla), Bornean orangutan (Pongo pygmaeus), and Sumatran orangutan (Pongo abelii).

There are six species of great apes. The other two species – the chimpanzee (Pan troglodytes) and the bonobo (Pan paniscus) – are endangered.

G. berengei includes two subspecies. One of them, Grauer’s gorilla (G.b. graueri), has experienced a decline in population of nearly 80 percent since 1994. There are about 3,800 individuals left. The other, the Mountain gorilla (G.b. beringei), has a population of about 880 individuals.

“To see the Eastern gorilla – one of our closest cousins – slide towards extinction is truly distressing,” Inger Anderson, IUCN’s director general, said in a statement. “We live in a time of tremendous change and each IUCN Red List update makes us realize just how quickly the global extinction crisis is escalating.”

The great apes are man’s closest relatives in the natural world.

Chimpanzees and bonobos share about 98.8 percent of the human genome. Gorilla genes are about 98.4 percent identical to humans, while the orangutan genome is about 97 percent identical to man.

The IUCN announcement came at its annual conservation congress, a gathering of political leaders, conservationists, and others, in Honolulu, Hawaii.

An infant Grauer’s gorilla is carried on the back of an adult. Photo courtesy Wildlife Conservation Society, photo by A.J. Plumptre.

The eastern lowland gorilla, Earth’s largest primate, is in rapid decline and has seen its population decline by nearly 80 percent since the late 1990s. There are now fewer than 4,000 individuals of the subspecies remaining in the wild.

Persistent war in the animal’s home range, which is limited to a forested region in eastern Democratic Republic of Congo, is the leading culprit for the rapid extermination of the great ape species.

“Since 1996, the entire range of Grauer’s gorilla has been consumed in conflict,” the report said. “This has resulted in an almost complete breakdown in government control, including wildlife protection activities.”

The civil war in DRC began in in the aftermath of the 1994 genocide event in Rwanda. Hundreds of thousands of refugees streamed into DRC. Upon arrival, they engaged in deforestation in the eastern region of the country. The inflow of Rwandans refugees also helped set off a conflict that killed millions of people between 1996-2003.

Although the war is over, the militias who participated in it have not disappeared. They control areas in the eastern DRC that are the only habitat for Grauer’s gorillas and, in that territory, they tolerate mining and engage in bushmeat hunting.

The mining, which is done on a small scale and often illegally, is aimed at extracting minerals used in the production of elecronic devices such as cellular phones, laptop computers, and gaming consoles.

Grauer’s gorillas, as well as chimpanzees, are hunted by the militia soldiers to feed the miners, which fund them, and themselves. Although protected by law, the large size of a Grauer’s gorilla means it can provide enough meat to feed multiple humans. Because the animal moves in a troop through its forested habitat, hunters can take multiple gorillas and feed even more humans.

Disarming the militias and imposing legal controls on the small-scale mining within Grauer’s gorilla habitat is a crucial step toward assuring the subspecies’ survival, said the report’s authors.

“Significantly greater efforts must be made for the government to regain control of this region of DRC,” Andrew Plumptre, a wildlife biologist with Wildlife Conservation Society and the lead author of the report, said. “In particular, the government needs to quickly establish Reserve des Gorilles de Punia and the Itombwe Preserve, and reinforce Kahuzi-Biega National Park efforts, which have community support, and to establish strong communication between [Institut Congolais pour la Conservation de la Nature] and the DRC military to tackle armed militias that control mining camps in Grauer’s gorilla heartland.”

The report also concluded that agriculture, poaching for body parts, and “socio-economic depression from over a decade of civil war” are contributing to the rapid decline of Grauer’s gorilla and other plant and animal species.

Stuart Nixon, a wildlife biologist at the United Kingdom’s Chester Zoo and a co-author of the report, emphasized that a speedy government response to these stressors is vital.

“Unless greater investment and effort is made, we face the very real threat that this incredible primate will disappear from many parts of its range in the next five years,” he said.

The most recent prior population survey of Gorilla beringei graueri occurred during the mid-1990s. Researchers concluded that a population of about 17,000 individuals remained at that time.

The current conflict is not the first occasion in which Grauer’s gorillas have suffered extensive losses at the hands of humans.

During the 1960s and 1970s many individuals were killed as grassland areas of their range were converted to agriculture and farmers used shotguns provided by the government of Zaire to kill the gorillas.

Gorilla beringei graueri is one of four gorilla subspecies. Like individuals of the other three subspecies, Grauer’s gorillas live in groups. They are thought to organize themselves into harem-like assemblages that include two males. A female matures at about eight years of age, while a male reaches full development after about 12 years.

A full-grown male Grauer’s gorilla can weigh up to 400 pounds.

Both females and males leave the group at maturity, with male Grauer’s gorillas staying together until each can attract females and form new groups. Females join a group or ally themselves with a single adult male.

Also known as the eastern lowland gorilla, Grauer’s gorilla is closely related to the smaller western lowland gorilla (Gorilla gorilla gorilla) that is endemic to central African forests.

The report’s authors recommend that the status of Gorilla beringei graueri be downgraded from endangered to critically endangered by the International Union for the Conservation of Nature.

A species listed as “critically endangered” is just one step, on the IUCN hierarchy of classification, from extinction in the wild.

The report was published by the Wildlife Conservation Society, Flora and Fauna International and the Congolese Institute for the Conservation of Nature.

Anyone who has seen chimpanzees in a zoo know that the African primate has a tendency to throw things at people: Rocks, for example, or poop.

Now research shows that Pan troglodytes also likes to throw rocks at trees.

This image shows stones accumulated at the base of an African tree. They were thrown by chimpanzees. Photo courtesy Max Planck Institute, Institute for Evolutionary Anthropology; Pan-African Program: The Cultured Chimpanzee; Chimbo Foundation.

Scientists collected data at 39 sites in four African countries for 14-17 months since 2010. They used non-invasive methods, including camera traps, to demonstrate that chimpanzees engage in a behavior that involves picking up stones near or in a tree and then launching them at the tree.

The behavior is not limited to one or the other gender. The researchers concluded that males most commonly engaged in it, but females also throw rocks at trees. So do juveniles.

It is not clear why chimpanzees throw rocks at trees.

One possibility proposed by the researchers is that it is a form of male dominance display. Another is that it is an expression of chimpanzee culture.

“As the stone accumulation behavior does not seem to be linked to either the abundance of stones or the availability of suitable trees in an area, it is likely that it has some cultural elements,” Christophe Boesch, director of the Department of Primatology at the Max Planck Institute for Evolutionary Anthropology, said.

Chimpanzees are known to use tools such as sticks to extract fish or ants or to remove honey from hives. They also communicate with each other, including by drumming on tree roots and by vocalizing pant hoots.

Pan troglodytes is Homo sapiens‘ closest relative. An individual human’s genome is 99 percent similar to that of a chimpanzee.

Social animals who live in groups that range in size from as few as five to as many as about 150 individuals, chimpanzees are omnivorous. They are known to eat fruit, bark, leaves, and stems, as well as other mammals. Colobus monkeys are a particularly common prey animal for chimpanzees.

Pan troglodytes is an endangered species under American law. Their population is thought to have declined from more than a million at the turn of the twentieth century to as few as about 200,000-300,000 individuals at present, according to FWS.

Chimpanzees, like the mother and infant pictured here, are humans’ closest living relatives. Flo, the female adult in the image, and her infant Flint were made famous when the primatologist Jane Goodall wrote of her research involving the two chimpanzees. Photo courtesy Jane Goodall Institute.

The new paper appears in the Feb. 29 edition of Nature Scientific Reports.

Bornean orangutans, like their Sumatran cousins, are arboreal and depend primarily on fruit for food. Courtesy Wikimedia.

Orangutans are gentle animals. Among Homo sapiens‘ closest relatives, the arboreal red apes from Asia got their name because the humans who discovered them thought they were “people of the forest.”

They are not immune from a willingness to commit violence. Conflict among male great apes is relatively common and male orangutans will rape females. Among females, fights that cause severe injury or death are unusual.

Now researchers have, for the first time, seen a female orangutan attack and kill another female. She didn’t act alone, either. Her male consort helped.

Orangutans do not live in groups. They tend toward the solitary. But females can have overlapping home territories. In the case described in a recent paper published in the journal Behavioral Ecology and Sociobiology, a young female basically arranged the death of an older female.

The events occurred in 2014. The younger orangutan, named Kondor, and the older orangutan, Sidony, had clashed some years earlier. This time, Kondor enlisted the help of a male named Ekko, who was not yet mature enough to have the characteristic male cheek flanges. Kondor and Ekko copulated near Sidony, then Kondor broke off the sexual encounter and attacked the older female. Ekko prevented Sidony’s escape and also bit her repeatedly. An older male eventually intervened to protect Sidony, but she suffered such extensive injuries that she died two weeks after the fight.

Orangutans are mostly fruit-eaters, though they will also eat bark, leaves, and insects. Their offspring are dependent on parents for a long time. An infant orangutan can be expected to nurse for six years and females stay with their mothers until they are teenagers.

The two species of orangutan – Pongo abelii and Pongo pygmaeus – live on Sumatra and Borneo and are among the most endangered animals on the planet.Their habitat – forest – is under assault by human demand for palm oil and both species are victimized by poaching.

The Orangutan Conservancy estimates that only about 40,000 individuals survive.